Bubble cooling

Bubble cooling is generally accomplished by blowing a large volume of air on the film as it exits the die (Fig. 3.10). This may take place on only the outside of the bubble or on both the inside and the outside. Additionally, the bubble is kept inflated to remove more heat from the film as it travels up through ambient air in the cooling tower. 

Bubble cooling deserves much attention. In many blown film operations, it is the limiting factor to maximizing throughput. Therefore, it is important to have a thorough understanding of the variables that influence the removal of heat from the bubble and ways to improve the efficiency of this process. 

Ambient air temperature around the extrusion line also has a large effect on bubble cooling, even when chilled air is used in the process. This is why frost-line height may change significantly from day shift to night shift in plants that are not air-conditioned. 

The principal hardware component responsible for cooling is the air ring. This device is located just on top of the die with a layer of insulating material (or air) between it and the hot die face (Fig. 3.12). The air ring surrormds the bubble and delivers cooling air directly onto the bubble. It receives air from the blower through, typically, a number of hoses that attach around the circumference of the device. Inside the air ring, a series of baffled flow channels distribute the air in such a way as to produce a uniform airflow (volume and velocity) at all points around the circumference of the bubble. 

An alternative technique for bubble cooling utilizes water applied directly to the outside surface of the bubble (Fig. 3.15). This process provides very high heat removal rates yielding significant output increases and reduced polymer crystallinity (clearer film). Although this technique was developed decades ago, it has seen very little commercial application, primarily due to its complexity. Recently, the process has been redeveloped for application to coextruded structures. 

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Rinse the oil-phase container with 50.0 g of nitrogen-bubbled cooled item 14 and transfer the rinsing to the manufacturing vessel. 

Na carboxymethyl cellulose, followed by heating to 50—60° (to remove air bubbles), cooling and mixing with 41.9ps AN 14.7 of a fine grade propellant]... 

V. Sidiropoulos and J. Vlachopoulos, Numerical Study of Internal Bubble Cooling in Film Blowing, Int. Polym. Process., 16, 48-53, (2001). 

Fig. 3 Schematic of spiral mandrel blown film die operation (1) ring-shaped melt distribution (2) die body (3) spiral flow mandrel (4) sizing ring (5) spreader (6) film bubble (7) frost line (8) solidified film (9) bubble collapsing rollers (10) nip rollers (11) external bubble cooling air (12) internal bubble cooling air inlet (13) internal bubble cooling pipe and (14) heated internal bubble air return.

The properties of the film are determined by the blow-up ratio and the speed. The blow-up ratio is the ratio between the diameter of the final tube of film and that of the die. The internal air pressure that expands the tube into the bubble is typically supplied through a port into the mandrel, the interior part of the die. Once the process is running steadily, little air is usually lost, so make-up requirements are small. When internal bubble cooling is used, air is constantly being exchanged inside the bubble. 

IBC internal bubble cooling ILD indentation load deflection... 

Bubble Breathing occurs when the air volume inside the bubble increases and decreases periodically. This is primarily a problem with internal bubble cooling (IBC) systems, since there is an internal air exchange occurring continuously. Solutions generally focus on checking IBC valves, blowers, and sensors. 

Uneven die lip temperature produces similar results to uneven bubble cooling. If one position at the die gap is hotter than other positions, the polymer will flow more easily through that area, yielding thicker film in that region. Again, this effect is exploited by some automatic thickness control systems that employ heaters embedded near the die lips to adjust flow rates by position in an effort to ensure thickness uniformity. 

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